My understanding is that they are not chemically the same: you can't smoke cocaine or amphetamine salts; they'll burn instead of vaporizing. You need to first swap what stabilizer the active component is bound to. i.e. perform some reaction to convert to crack/meth. The point of doing that conversion is to be able to smoke it. My understanding is it's also not just fast acting, but quite a bit more intense. So e.g. more likely to trigger psychosis instead of just making you talk a lot, clean the house, get good grades, etc.